The present invention relates to a method for implementing PVT control of an injection molding operation wherein the pack time and hold time are dynamically adjusted based on sensing parameters related to melt temperature.
In an injection molding process, solid polymer is melted, typically in a rotating screw injection unit, forced into a closed mold in which it solidifies, thereby assuming the geometry of the mold cavity. Although this process at first appears to be simple, in practice, an experienced molder knows that the successful application of the process is critically dependent on a very elusive complex of interrelated dimensions: mass, time, pressure and temperature. A change in any one of these machine parameters modifies one or more of the other dimensions and results in changes in the molded article.
Process engineers are well aware that variability and operating parameters during injection will result in changes in the molded article. A very significant parameter is the temperature of the melt during injection, which affects the density of the molded parts. In the injection molding process, once the mold is completely filled, additional plasticized material is packed into the mold by continuing to exert injection pressure until the gate freezes. Accordingly, if there is a higher melt temperature but the same amount of packing pressure, the density of the material will be lower because there will be less mass in the mold. Accordingly, in order to pack more material into the mold at higher melt temperature, packing must occur at a higher pressure or the pressure profile must be for a longer period of time in order to ensure that the mold is filled properly.
If the amount of material packed into the mold changes during a run, different size parts will result at atmospheric pressure when the part has cooled to room temperature. This is highly undesirable in most cases, because a part that is too dense will consume more raw material, and if the density is not according to specification, the performance of the part may be unacceptable.
Although the variability in operating parameters results in changes in the molded article, it is often not apparent, without the advantage of extensive testing, just how much the part has changed in response to a given parametric variation. To achieve a desired part characteristic, the determination of the number of relevant parameters requiring adjustment, as well as the correct direction and amplitude of adjustment, is an art currently practiced with difficulty.
The objective of all process controls is the optimization of the molded part in accordance with certain set-up specifications. Currently, three main categories of injection molding control systems are in use: open loop, close loop and adaptive control. Adaptive control processing, which is the newest category of control systems; is a sophisticated approach to injection molding process management in which predetermined set points are "intelligently" modified, based on variations in process parameters. PVT (pressure, specific volume and temperature) optimization is probably the most sophisticated process control system developed for injection molding to date. However, a practical disadvantage of such an approach is the necessity for accurate melt temperature measurements in order to profile holding pressure and cooling time.
The manner in which pressure, volume and temperature are interrelated and the various phases through which a shot of resin goes during an injection molding cycle incorporating PVT holding pressure optimization are represented schematically in the tear drop-shaped PVT diagram shown in FIG. 2. The PVT optimization routine, which is described in the literature, is concerned with pressure, specific volume and temperature as well as the mean temperature profile in the mold, as determined by a cooling calculation. In the diagram of FIG. 2, pressure values are represented by diagonal isobar lines 10 wherein the uppermost line is at atmospheric pressure and the lines below it indicate successively higher pressures. Specific volume is defined as the volume of plastic per unit weight and is the inverse of its density. The objective of PVT control is to return the molded parts to the same temperature at atmospheric pressure after cooling, which assures that the specific volume, and, therefore, the density and part weight, will be consistent. In other words, the temperature and specific volume of the material starts at point 12 on the diagram of FIG. 2, is plasticized, injected and cooled and returns to the influence of PVT optimization to the same atmospheric isobar, at which point it cools down to room temperature at point 12 on the diagram.
During injection when the mold is being filled, which is represented by line 14 on the tear shaped curve, the mold is being filled and the material is pressurized, which results in the material remaining essentially at the same temperature, which results in generally a vertical drop in specific volume until point 16 is reached, at which point the pressure is established. At this point, the mold is filled and the system then switches over to a holding pressure to provide holding or packing of the material in the mold, and this is represented by line 18 on the diagram of FIG. 2. It will be noted at this point, the curve moves down along the same isobar to point 20, at which point the gate freezes so that whatever mass of material was packed into the mold at that point is the final fill quantity of the mold.
At this point, temperature decreases because the mold is completely filled and the specific volume is nearly constant as the curve moves to the left back to point 22, which is atmospheric pressure.
If the melt is at a higher temperature following plasticating, this changes the position of line 14 on the diagram of FIG. 2, essentially moving that line 14 to the right. This also changes the location of point 16, which must be on the same pressure isobar, but will result in greater specific volume for the shot. In order to pack and hold the shot and return to point 22, a longer cooling time will be necessary because there is a greater temperature differential between point 16 and point 22. If this is done, then the specific volume of the shot will be the same as in the previous shot and the part density will be uniform.
The PVT optimization control system is based upon physical data from molded parts judged to be optimum, and the desired specific volume at atmospheric pressure must be predetermined. Present PVT optimization systems require thermocouples in both halves of the mold, a cavity pressure transducer and melt temperature sensor in the nozzle region. Maximum cavity pressure is user-selected to minimize the possibility of mold flashing or damage. The holding time may be calculated using a cooling equation, such as the Ballman and Shusman formula: EQU t.sub.c =(S.sup.2 /.pi..sup.2 a)ln [(T.sub.M -T.sub.m /.pi..sup.2 (T.sub.E -T.sub.m)]
Wherein:
T.sub.M =Melt Temperature PA1 T.sub.m =Mold Temperature PA1 s=Part Thickness PA1 T.sub.e =Temperature at Injection PA1 a=Thermal Diffusivity PA1 t.sub.c =Cooling Time
As is evident, cooling time is a function of mold and melt temperatures, part geometry and material thermal characteristics.
A significant problem with the classical PVT approach to injection molding process control arises from the requirement for continuous melt temperature monitoring. The holding pressure profile is continuously adjusted in accordance with changes in melt temperature, and the temperature sensors, which are located in the nozzle or in the front of the barrel, are subjected to very high pressures and temperatures. Radiant temperature monitoring devices are very susceptible to abuse in the normal molding environment, and under manufacturing conditions, precision monitoring of melt temperature in the nozzle ranges from difficult to impractical. Although PVT process control has proven quite successful in achieving uniform part density, the high cost, maintenance problems and short life of melt temperature sensors have severely limited its commercial viability.